1. The Field of the Invention
This invention relates to control means for opening and closing a valve in the feed water line of a membrane separation system, actuated by pressure in a permeate storage tank.
2. The Prior Art
Many arrangements have been proposed and used by the prior art for automatically controlling operation of a reverse osmosis system dependent on the pressure in a permeate storage tank. The permeate may be transferred from the reverse osmosis module to a storage tank which contains a compressible fluid, such as air, separated from the permeate by a flexible diaphragm such as described in U.S. Pat. No. 3,493,496. If permeate is not withdrawn from the storage tank, and the reverse osmosis unit continues to operate, permeate pressure continues to rise until it reaches the feed pressure, or is relieved, for example, through a midsection tap in an elongated conduit restrictor controlling brine release, as described in U.S. Pat. No. 3,568,843. Such relief, which operates by draining off excess permeate through the brine control and disposal system, works very well, but does not shut down the reverse osmosis system, and feed water continues to flow and the excess permeate is wasted. For conservation of water, a control which shuts off the feed water flow when the permeate tank pressure reaches a predetermined control value is more desirable, for example, as shown in the U.S. Pat. Nos. 3,939,074 and 3,887,463.
Automatic valves actuated by pressure in the permeate storage tank are effective, but may at times tend to stick or bind especially at the low end of their operating range where the control range is narrow. Valves actuated by external power, such as solenoid valves, may be employed, but these require connection to a source of electric power and to an electric reversing or control switch. At the shut-off end of the range where the pressures are higher and the control more positive, automatic operation may be more satisfactory. At the re-open end of the range, however, where pressures are lower, automatic operation may be less positive. Control parts may stick and storage tank pressure may fall substantially below the desired re-open pressure before valve actuation is accomplished.
A simple automatic valve control system that would provide more positive feed water control, particularly at the re-open end of the operation range, would be valuable.